Modern electronic components are used in a wide variety of environments. Many electronic components are subject to corrosion resulting from environmental factors, such as water vapor. Such components are, therefore, typically sealed in dry nitrogen in an airtight package. Such packages typically comprise a box-like base, to which the electronic components are mounted. The sidewalls may be provided with feedthrough holes through which conductive pins are mounted to electrically connect electronic components inside the package to systems outside the package. Such pins are electrically insulated from and hermetically sealed to the package sidewall(s). After the electronic components are mounted in the package, a cover is hermetically sealed to the base along the upper rim of the sidewalls.
The materials composing the package are carefully selected to meet a number of application specific requirements for density, thermal expansion, thermal conductivity, and mechanical strength. For example, packages used on aircraft or spacecraft must be lightweight to preserve airworthiness, fuel efficiency, and maneuverability and, therefore, comprise low density materials. Packages used in high electric current applications should be constructed from materials having high thermal conductivity, so that heat generated within the package is conducted outside to maintain a lower temperature inside the package. The service life of components is generally enhanced when lower temperatures are maintained within the package.
Electronics packages should generally be constructed from materials having a thermal coefficient of expansion ("TCE") approximately equal to that of materials the packages contact. Otherwise, temperature changes produce stress and warping as the package and contacting materials expand and contract at different rates. Because electronic components are often mounted on low TCE ceramic chips inside the package, most electronic packaging applications require a material having a low TCE generally matching that of the ceramic chips. Packages made from ferrous alloys, such as Alloy 52 or KOVAR, have a low TCE but are relatively heavy. Packages made from aluminum are light in weight, but the TCE of aluminum is much higher than that of the ceramic chips.
In general, materials having a low density, low TCE, high thermal conductivity, and good mechanical strength are desirable for electronics packaging applications. One type of material that exhibits such desirable properties is a metal matrix composite ("MMC"). Such materials contain a non-metal reinforcing material dispersed within a metal matrix, or host material. The reinforcing material is typically in the form of fibers or particulates. The TCE, density, thermal conductivity, and mechanical strength can be manipulated by carefully selecting the matrix material and the form, percentage, and composition of the reinforcing material. For example, an aluminum alloy matrix with a silicon carbide particulate reinforcement has low density, low TCE, good thermal conductivity and satisfactory mechanical strength.
Metal matrix composite materials are well suited for electronics packaging applications. Difficulties have been encountered, however, in hermetically sealing the cover and feedthrough pins to MMC packages. Conventional non-MMC packages made from aluminum or ferrous alloys are typically sealed using a resistance welding process known as seam welding, or using laser welding. MMCs generally cannot be laser welded, however, because of the difference in energy absorption rates between the metal matrix material and the non-metal reinforcing material. Seam welding is generally unsuitable for non-ferrous materials such as aluminum.
Soldering or brazing a cover onto a package base tends to heat and expand the nitrogen in the package, inducing a partial vacuum within the sealed package as the nitrogen cools to room temperature. The vacuum causes structural stresses and can deform the package. Furthermore, the heat generated using soldering or brazing techniques to seal the cover to the package base raises the temperature inside the package, which can jeopardize the components housed within. None of the traditional sealing methods is thus suitable for hermetically sealing MMC electronics packages.
One solution for sealing a cover onto an aluminum matrix MMC package is to fabricate a package base in which the upper sidewall rims are free of the reinforcing material, as described in "Investment Cast Metal Matrix Composite," S. Kennerknecht, Society of Manufacturing Engineers Technical Paper EM90-441, 1990, available from the Society in Dearborn, Michigan. For example, an aluminum/silicon carbide base can be cast using a preform, i.e., a structure inserted into a mold to become part of the final casting, to form an upper rim of the base sidewalls comprising a different material, such as aluminum. A cover, preferably comprising aluminum, can then be laser welded to the aluminum rim.
Such a package comprising an MMC base with a metallic rim tends to warp upon cooling, however, as a consequence of the difference between the TCE of the MMC base material and that of the metallic rim. Although warpage of small packages can be corrected after cooling using an additional machining operation, the packages will tend to warp again as the temperature of the package fluctuates during operation. Therefore, this arrangement is useful only for very small packages in which the resulting warpage is minor enough to be acceptable. Larger component packages and packages with large length-to-width ratios tend to warp severely and thus cannot be successfully sealed using an unreinforced metallic preform.
In short, although materials such as MMCs that have physical properties meeting the requirements of electronics packaging applications are available, techniques for providing hermetically sealed MMC packages have not yet yielded satisfactory results.
An object of the invention is, therefore, to provide hermetically sealable electronics packages having desirable weight, thermal expansion, thermal conductivity, or mechanical strength properties suitable for a wide variety of applications.
Another object of the invention is to provide such electronics packages at competitive production costs.
A further object of the invention is to provide hermetically sealable electronics packages composed of an MMC material.
Yet another object of the invention is to provide a method for producing and hermetically sealing electronics packages made from a wide variety of materials, including MMCs.